Method of making thermal recording print head

ABSTRACT

A thermal print head and a method of manufacturing it using a thermally stable support mandrel and a plurality of spaced turns of copper-coated resistance wire wrapped on the mandrel. The turns are held on the mandrel by slabs of potting compound retained in under-cut grooves on the mandrel while the connections to the print elements are made by slitting the turns along a longitudinal groove in the mandrel. The copper coating is removed to expose the resistance wire at the desired printing element locations.

United States Patent 1 1 Lane, III

[ June 24, 1975 METHOD OF MAKING THERMAL RECORDING PRINT HEAD [75]lnventor: Charles E. Lane, III, Meadowbrook,

[73] Assignee: Honeywell, Inc., Minneapolis, Minn.

[22] Filed: July 22, 1974 [21] Appl. No.: 490,230

Related US. Application Data [62] Division of Ser. No. 457,622, April 3,1974.

[52] US. Cl. 29/611 [51] Int. Cl. H05b 3/00 [58] Field of Search 29/611,629, 597, 610;

[56] References Cited UNITED STATES PATENTS 2,703,917 3/1955 Pankove29/253 3,578,946 5/1971 Colello 3,781,983 l/1974 Hiruma et a1 29/6113,795,884 3/1974 Kotaka 29/629 X FOREIGN PATENTS OR APPLICATIONS 525,9912/1954 Belgium 29/611 Primary ExaminerC. W. Lanham AssistantExaminerVictor A. Di Palma' Attorney, Agent, or FirmArthur H. Swanson;Lockwood D. Burton; Mitchell .1. Halista [5 7 ABSTRACT A thermal printhead and a method of manufacturing it using a thermally stable supportmandrel and a plurality of spaced turns of copper-coated resistance wirewrapped on the mandrel. The turns are held on the mandrel by slabs ofpotting compound retained in under-cut grooves on the mandrel while theconnections to the print elements are made by slitting the turns along alongitudinal groove in the mandrel. The copper coating is removed toexpose the resistance wire at the desired printing element locations.

3 Claims, 5 Drawing Figures 1 METHOD OF MAKING THERMAL RECORDING PRINTHEAD This is a division of app'lication'S er. No. 457,622 filed on Apr.3, 1974. t

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention is dir ected to a thermalprinter' print head and to a methodfor manufacturing such a head. More specifically, the present inventionis directed to a thermal printer print head, for thermal recording on aheat sensitive recording medium and for mounting on a printed circuitboard. Thermal print heads may be found in Class 219 Electric Heating,Subclass 216 of the United States Patent Off ce classification system. I

2. Description of the Prior Art In order to obviate theproblemassociated with conventional recording techniques in recordinginformation, particularly from data processing systems, thermalrecorders have been developed forrecording on a thermally responsiverecording medium. Forexample, such thermal recorders obviate the use. ofan inking pen which type of pen is undesirable for many recorderapplications because of problems associated with clogging of the pen aswell as the need to replenish the ink supply. Similary, the thermalrecorder is free of the problems associated with a high voltage type ofrecording wherein recording electrodes using an electric arc to mark arecording medium are exposed to arc erosionwhile the arc imposeslimitations on the environmental exposure of the recording apparatus,e.g., an explosive atmosphere. Further, the thermal recorder can be usedfor recording analog and digital information on the same recordingmedium. Conventional thermal recording devices are shown ,in U.S. Pat.Nos. 3,145,071; 3,312,979; 3,334,352; 3,354,817; 3,453,647; and3,718,564. While these prior art thermal recorders are capable ofrecording on a thermally responsive medium, none of the aforesaidrecorders employs a recording head which is capable of being assembledby an automatic manufacturing technique. Further, a practical thermalprint head must lend itself for use with printed circuits to minimizethe manufacturing costs thereof and to produce a miniaturized print headcapable of operating as a high speed printing element, i.e., a lightweight printing head having minimal thermal inertia, or heating andcooling time.

SUMMARY OF THE INVENTION Accordingly, it is an object of the presentinvention to provide an improved thermal print head adapatable forautomatic manufacturing techniques while exhibiting high speed recordingcapabilities in a miniaturized assembly.

Another object of the present invention is to provide a novel method ofmanufacturing a thermal print head.

In accomplishing these and other objects, there has been provided, inaccordance with the present invention, a thermal print head having aplurality of spaced resistance heating elements supported on a firstlongitudinal edge of a thermally stable triangular crosssection supportmandrel. A longitudinal undercut groove is located in each face of thesupport mandrel adjacent to this first longitudinal edge with a firstfree end of each of the heating elements passing across a first one ofthe longitudinal grooves and a second free end of each of the heatingelements passing across a second one of the longitudinal grooves. Aretainer means is located within each of the grooves and is arranged toencompass the portion of the heating elements passing acrossacorresponding groove to retain the heating elements on the firstlongitudinal edge and to fill the corresponding longitudinal groove. Theprint head is manufactured by a method including the steps of winding aresistance heating wire having an electrically conductive coating withspaced turns on the mandrel with each turn of the wire passing acrossthe first longitudinal support edge of the mandrel to form a thermalprinting element at the apex of the first longitudinal support edge, andmoulding a slab of retaining material around the portions of the wirespassing over the undercut grooves while filling the undercut groove bythe retaining material to be retained therein. The wire turns are cutalong a third longitudinal groove located in a third face of the mandrelto form free wire ends for each printing element suitable for providinga connection to energizing circuits for the print head.

BRIEF DESCRIPTION OF THE DRAWING A better understanding of the presentinvention may be had when the following detailed description is read inconnection with the accompanying drawings in which:'

FIGS. 1 through 4 are step-by-step illustrations ofa sequentialmanufacturing techniques embodying the present invention for producing anovel thermal recording head, and

FIG. 5 is a cross-section of an assembled and mounted novel thermalrecording head formed by the method shownin FIGS. 1 to 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT DETAILED DESCRIPTION Referringto FIGS. 1 to 4 in more detail, there is shown a sequentialmanufacturing operation embodying the present invention for producing athermal recording head having a plurality of thermal recording elementsin an integral arrangement. Starting with FIG. 1, there is shown athermally stable support mandrel 2 having a triangular cross-section. Apair of longitudinal undercut grooves, or dovetails, 4 and 6 areprovided in adjacent faces 2A and 28, respectively, of the supportmandrel 2 which faces 2A and 2B join along one edge to define alongitudinal support edge 8. A third face 2C of the mandrel 8 is alsoprovided with a longitudinal groove 10 suitable for attaching thethermal print head to a support member, as discussed hereinafter. Thesupport mandrel 8 may be an aluminum bar which is hard-anodized andcoated with a suitable material, e. g., tetrafluorethylene, to provideelectrical insulation for the aluminum base member. The mandrel 8 iswound with a single layer of spaced turns of an electrically insulatedand copper plated resistance wire 12 with the spaced turns of thewinding passing across the first longitudinal support edge 8. This wireconsists of a core of resistance wire which is copper plated and coatedwith solder-through, i.e., heat-meltable, electrical insulation toenable the resistance wire to be connected to a support member, asdiscussed hereinafter.

As shown in FIG. 3, a wire retaining member in the form ofa slab ofapotting compound 14, I6 is moulded in place on the outside surface ofeach of the faces 2A and 2B having the aforesaid undercut longitudinalgrooves 4 and 6, respectively. therein. This potting compound 14, 16 isarranged to have the turns of the resistance wire 12 embedded thereinwhereby to retain the wires on the mandrel 2 in a fixed spacedrelationship while the portion of the potting compound extending intothe undercut grooves 4 and 6 provides a means for locking the pottingcompound 14, 16 to the mandrel 2. Subsequently, each of the turns of theresistance wire 12 is cut by passing a cutter (not shown) along thegroove located in the third face 2C of the mandrel 2. This cuttingoperation is effective to provide a pair of free ends for each of thesegments of the resistance wire passing across the aforesaid firstlongitudinal support edge 8, as shown in FIG. 4.

The completed thermal print head is attached to a support member, suchas a printed circuit board 18, by self-tapping attaching screws 20passing through the circuit board 18 into the longitudinal groove 10 inthe third face 2C of the mandrel 2. The width and depth of the groove 10is selected to provide suitable space for the self-tapping attachingscrew 20. The free ends of the resistance wire heating elements areattached to platedthrough holes 22 in the printed circuit board 18 byany suitable soldering technique which removes the solder-throughinsulation from the wires and connects the copper-plating to theplated-through holes 22. Each of the plated through holes 22 is, inturn, connected to printed wiring 24 located on the printed circuitboard 18 for conducting an energizing signal to a corresponding one ofthe thermal printing elements and may, further, be connected to otherassociated equipment located on the printed circuit board 18 to minimizeextraneous electrical connections.

The exposure of the core of the resistance wires forming each of theprinting areas at the apex of the first longitudinal support edge 8 maybe achieved either before or after attaching the thermal print head tothe printed circuit board 18 by etching the wires to a predetermineddistance on either side of the first support edge 8. This etchingprocess using, for example, a solution of ferric chloride, is arrangedto remove the solderthrough coating and the copper plating on theresistance wires while not affecting the electrical insulation of thesurface of the mandrel 2. Thus, the resistance wires will heat up onlyin the area where the copper plating has been removed since the copperplating will serve as a conductor along other portions of the resistancewires to bypass the internal core of resistive material. Thus, theamount of material to be heated to form a thermal print character islimited to the etched area on the first longitudinal support edge 8.

Accordingly, it may be seen that there has been provided, in accordancewith the present invention, a novel thermal print head and a novelmethod for producing the thermal print head in an automaticmanufacturing operation to produce a plurality of thermal printingelements in a integral constuction.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. A method of manufacturing a print head comprising the steps ofwinding a single layer of spaced turns of a wire on an electricallyresistive support mandrel having a longitudinal wire support edge and anundercut groove in each face of said mandrel adjacent to said supportedge, filling said grooves in said mandrel and encompassing adjacentones of said wires with a wire retaining compound and cutting saidwinding of wire along a predetermined longitudinal line on said mandrel.

2. A method as set forth in claim 1 wherein said wire is a resistancewire having an electrically conductive coating thereon and including thefurther step of removing said conductive coating at said support edge toleave said resistance wire passing across said support edge.

3. A method as set forth in claim 1 and including the further step ofattaching the free ends of said wire turns adjacent to said longitudinalline to respective printed wires on a printed circuit board.

1. A method of manufacturing a print head comprising the steps ofwinding a single layer of spaced turns of a wire on an electricallyresistive support mandrel having a longitudinal wire support edge and anundercut groove in each face of said mandrel adjacent to said supportedge, filling said grooves in said mandrel and encompassing adjacentones of said wires with a wire retaining compound and cutting saidwinding of wire along a predetermined longitudinal line on said mandrel.2. A method as set forth in claim 1 wherein said wire is a resistancewire having an electrically conductive coating thereon and including thefurther step of removing said conductive coating at said support edge toleave said resistance wire passing across said support edge.
 3. A methodas set forth in claim 1 and including the further step of attaching thefree ends of said wire turns adjacent to said longitudinal line torespective printed wires on a printed circuit board.